My oldest did this until he was....6 I think? He had no physical problems that could be seen, no speech problems, nothing. At your child's age, it is very common for the parents to be ale to understand almost everything and others to be able to understand only about half of what the child is saying.

My oldest was (still is) a VERY oral child. He puts things in his mouth more than his 10-month-old teething sister. Teething necklaces did nothing, in fact he pulled on it so hard to try to mouth it, that he broke it. We have debated doing consequences for it, just to stop the cycle because as a result he seems to be sick ALL the time (big surprise), but it seems like such a silly thing to "punish". He seems to be outgrowing it anyway.